Engine-driven machines such as, for example, dozers, loaders, excavators, motor graders, and other types of heavy equipment typically include a cooling system that cools the associated engine and other machine components below a threshold that provides for longevity of the machines. The cooling system typically consists of one or more air-to-air or liquid-to-air heat exchangers that reduce a temperature of the coolant circulated throughout the engine or combustion air directed into the engine. Heat from the coolant or combustion air is passed to air from a fan that is speed-controlled based on a temperature of the engine and/or associated hydraulic system.
The cooling system fan is generally hydraulically powered. That is, a pump driven by the engine draws in low-pressure fluid and discharges the fluid at elevated pressures to a motor that is connected to the fan. The pump and the motor are generally connected in a closed loop configuration in which the pressurized fluid used to drive the motor is returned to the pump. When a temperature of the engine is higher than desired, the pump and motor work together to increase the speed of the fan. When the temperature of the engine is low, the pump and motor work together to decrease the speed of the fan and, in some situations, even stop the fan altogether. Under some conditions, the fan rotation can be reversed such that airflow through the heat exchanger is also reversed to help dislodge debris that has collected in the heat exchanger.
Although effective at cooling the engine and reducing engine speed, it has been found that the hydraulic circuit driving the cooling fan described above may have excess capacity at times that is not utilized. With increasing focus on the environment, particularly on machine fuel consumption, it has become increasingly important to fully utilize all resources. As a result, some machines attempt to utilize the excess capacity of the hydraulic fan circuit to power other fluid circuits associated with the machine. Such circuits may include open loop hydraulic circuits, such as braking systems and/or steering systems of the machine.
One such system is described in U.S. Pat. No. 7,360,357 to Zahniser, issued on Apr. 22, 2008 (“the '357 patent”). Specifically, the '357 patent describes a hydraulic system including primary and secondary fixed displacement pumps. The primary pump is configured to direct hydraulic fluid to a power steering circuit of a vehicle, and the secondary pump is configured to direct fluid to a fan motor used to drive a cooling fan. The secondary pump is fluidly connected to the fan motor in a closed loop configuration.
Although the hydraulic system of the '357 patent may be capable of operating both a fan motor and a power steering circuit associated with the vehicle, the disclosed system may increase the cost of the vehicle and may not be as durable as other known systems. That is, the system of the '357 utilizes separate pumps to operate the fan and steering circuits. The use of such duplicate components adds cost and complexity to the system. In addition, the fixed displacement pumps used in the system of the '357 patent may undergo rapid acceleration and/or deceleration at start-up, shut-down, and/or during a reversal of fan direction. Such rapid changes in pump operation may result in pressure spikes that, over time, may damage the pumps and/or other components of the hydraulic system.
The disclosed hydraulic fan circuit is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.